Gravel pack method

ABSTRACT

A gravel packing method wherein a viscous fluid is injected into an unconsolidated formation at such a rate and pressure to form a cavity therein and a specially graded aggregate is pressure packed in the cavity to form an outer sand exclusion zone and an inner high permeability zone. The high permeability of the inner zone permits the passage of for-mation fines which escape the sand exclusion zone.

United States Patent Wood et a1.

Goodwin l 66/276 GRAVEL PACK METHOD [72] Inventors: Donald B. Wood, Houston. Tex. 77005; Other M. Kiel, Houston,

Tex. 77035 Assignee: Esso Production Research Com- 3,559,736 2/1971 Bombardieri..........

Primary Examiner-Stephen .l. Novosad pany Attorney-John A. Reilly, John B. Davidson, Lewis H.

Eatherton, James E. Gilchrist, Robert L. Graham and James E. Reed [22] Filed: Jan. 16, 1970 ABSTRACT A gravel packing method wherein a viscous fluid is in- [21] Appl. No.: 3,412

1521b 43 04 ected into an unconsolidated formation at such a rate [5|] Int.

and pressure to form a cavity therein and a specially graded aggregate is pressure packed in the cavity to [58] Field of Search......................166/276, 278, 295

form an outer sand exclusion zone and an inner high [56] References Cited UNITED STATES PATENTS permeability zone. The high permeability of the inner zone permits the passage of for-mation fines which escape the sand exclusion mne.

5 Claims, 3 Drawing Figures 8888 7777 HRH 6666 6666 1111 mm m n flt e h hu mmmm RCCH 0 8 6H46 9999 111 WWW" 1 8 05 407 3 ,9 2762 5866 w v2. 2323 k so GRAVEL PACK METHOD BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a sand control method for completing wells in unconsolidated fonnations.

2. Description of the Prior Art In completing wells in poorly consolidated formations, consideration must be given to sand problems likely to arise during the operation of the well. The incompetent nature of this type of formation requires that the well completion technique include means for excluding sand production. The erosion and plugging effects of sand entrained in produced fluids are well known and if not arrested can seriously reduce well productivity.

The propensity of a formation to produce sand is particularly acute in formations characterized as unconsolidated or poorly consolidated. These terms, as applied to subterranean sedimentary deposits, define a particular class of sedimentary rock, the distinguishing characteristic of which is the absence of a matrix which binds the sand grains into a cohesive mass.

Analytical model studies have indicated that unconsolidated rock can also be distinguished on the basis of mechanical behavior when subjected to loading. The injection of a viscous fluid into an unconsolidated body at high pressures causes the body to deform in a plasticlike manner. Field tests have shown that when subjected to fracturing pressures subterranean unconsolidated formations deform in a plastic manner. In each of the wells treated the volumes of sand injected were considerably higher than that normally used in fracturing a consolidated formation. Moreover, the injection pressure for plastically deforming an unconsolidated formation was frequently higher than that normally required to fracture a consolidated formation.

A widely used sand control technique is the gravel pack installation which operates on the principle of forming a sand exclusion zone in surrounding relation to the wellbore. The sand exclusion zone composed of particularly graded aggregate screens out or bridges the formation sand entrained in the produced fluids. The typical gravel pack completion involves the placement of aggregate in the immediate vicinity of the wellbore and the provision of a support means for maintaining the aggregate in place. The aggregate is generally a specially graded sand or gravel, but can be other particulate material such as walnut shells, glass beads, and the like.

The placement of the aggregate in the formation presents a major source of trouble in performing the gravel pack installation. in open hole completions, the formation can be underreamed to form a large cavity for receiving the aggregate. [n perforated casing completions, the perforations can be washed to form a cavity outside the casing. The steps of forming the cavity and subsequently filling the cavity with the aggregate are time consuming, expensive, and frequently risky. MOreover this process cannot be used in slirn hole completions equipped with casing in the order of 2- /4. inches mainly because of limited downhole working space.

A recently developed pressure-pack technique avoids the necessity of washing or underreaming the formation and is therefore ideally suited for slirn hole 0 zone of considerable radial extent.

The main disadvantage of this type of completion is that the sand exclusion zone tends to become plugged with formation fines. The sand exclusion bridge provided by the aggregate does not prevent the migration of all the formation sands. Always present in the formation are grains of sufficiently small diameter to pass through the packed zone interstices. Unfortunately, these formation fines tend to accumulate and clog interstitial flow passages in the immediate vicinity of the wellbore. The plugging of interstices in this critical area can seriously reduce the well productivity.

SUMMARY OF THE INVENTION The present invention modifies the pressure-packed well completion by providing a well with two generally concentric packed zones: an inner zone surrounding the wellbore and an outer zone extending radially outwardly from the inner zone. The outer zone constitutes the sand exclusion zone and is packed with particularly sized aggregate for controlling migration of formation sands. The size of the aggregate in the sand exclusion zone can be determined by conventional techniques such as those based on the ill-percentile point on the sieve analysis curve. The inner zone, embracing the critical flow area is packed with a highly permeable aggregate, the particle size of which is selected to bridge the aggregate of the outer zone. Thus the relatively large size of the pore spaces in the inner zone will permit the passage of formation fines which escape the outer sand exclusion zone and thereby prevent the plugging of interstitial flow passages in the critical flow area adjacent the wellbore.

The completion method according to the present invention can be performed in the following sequence: deforming the formation by injecting a fluid therein to form a cavity adjacent the wellbore; and pressure packing the cavity with an aggregate to further displace the formation outwardly, the aggregate being graded to form an outer sand exclusion zone and an inner high permeability zone. In order to maintain the aggregate in place, a support is provided in the immediate vicinity of the wellbore. The support can be a mechanical device such as a perforated liner or can be provided by plastic consolidation. In regard to plastic consolidation, the present invention affords a more efficient application of the consolidating agents. While a reduction in permeability is generally associated with sand consolidation with plastics, the relatively high permeability in the inner zone can accommodate considerable reductions without adversely affecting well productivity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic sectional view of a well completed by one embodiment of the present inven tion;

FIG. 2 is a view similar to FIG. I illustrating a well completed by another embodiment of the present invention;and

FIG. 3 is a transverse sectional view of the well shown in FIG. I, the cutting plane taken through the completed interval.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in connection with perforated casing completions but it should be understood that the principles exemplified therein are equally applicable to other completions.

With reference to FIG. 1, a wellbore I is shown penetrating a massive unconsolidated formation 11 typical of those found in the Miocene sediments along the Gulf Coast areas of Texas and Louisiana. Characteristically, these formations lack a cementitious material which binds the sand grains into a cohesive framework. Consequently when the well is completed and placed on production the loosely held sand grains tend to migrate into the wellbore creating erosion and wellbore clogging problems.

After the well is drilled, a string of casing I2 is run to total depth and cemented in place. The casing 12 is then perforated in the productive interval. In ofi'shore operations it has been found more economical to use a single casing string having outside diameters as small as 2% inches. However, this practice presents serious operational problems because of limited downhole working space. For example, it is difficult to perform the conventional gravel pack operation which involves washing behind the casing and placing the production liner. The limited working space of the slim hole completion has been a major incentive in the development of the completion method according to the present invention. It should be observed, however, that the invention is not restricted to such completions.

Basically, the invention contemplates the pressure packing of a formation with a graded aggregate to provide an outer sand exclusion zone and an inner high permeability zone. Laboratory analytical model studies reveal that unconsolidated sand bodies behave differently from consolidated sand bodies when subjected to pressures commonly used in fracturing operations. These studies indicated that the unconsolidated sand behaved much in the manner of a plastic body. When the sand was subjected to a loading by the injection ofa substantially nonpenetrating fluid, it exhibited a plastic-like flow in the immediate vicinity of the loading force. In a sense then, the process can be considered a displacement process wherein the formation sands are displaced radially outwardly from the wellbore by a displacing medium. In recognition of this phenomenon, a method has been developed which not only provides a sand exclusion zone of relatively large radial extent but also provides a high permeability zone in the immediate vicinity of the wellbore, the critical flow area. In addition to providing a means of excluding sand, in some instances this process results in well stimulation, principally because of the increased permeability in the critical flow area.

Briefly, the method according to the present invention comprises initially injecting a fluid into the forma tion at such a pressure and rate to physically deform the formation forming a cavity adjacent the wellbore l0. and then pressure packing the cavity with a graded aggregate material to form an outer filter bed which provides a sand exclusion zone [3 and an inner filter bed which provides a high permeability zone 14. The aggregate suspended in a carrier fluid can be injected at sufficient rates and pressure to further expand the cavity. The aggregate screens out on the formation face as the carrier fluid filters into the formation 11 so that at the end of the placement step, a tightly packed zone surrounding the wellbore I0 is obtained. The pressure packing operation is performed in two stages: one stage for placing fine aggregate material to form the outer filter bed followed by the stage for placing a coarse aggregate material to form the inner filter bed. For purposes of illustration, the line aggregate for the sand exclusion zone 13 will be referred to as the pack sand and the coarse aggregate for the high permeability zone will be referred to as gravel.

After the formation is broken down, that is, when the formation begins receiving large quantities of fluid at a pressure gradient indicating physical deformation, a carrier fluid containing the pack sand is injected into the formation at about the same volumetric rate. The formation is further deformed as the aggregate screens out on the formation face. After a predetermined amount of pack sand has beeri placed, the gravel can be substituted for the pack sand in the carrier fluid and injection continued. Alternatively, the carrier fluid for the gravel can be different than the carrier fluid for the pack sand. The packed zone continues to grow in a generally radial manner until almost all of the slurry is displaced from the wellbore Ill. The final step in the process is to provide a support for maintaining the packed aggregate in place.

The equipment and technique for placing the aggregate in the formation can be the same as those conventionally used in fracturing operations. Initially, water can be injected down the casing 12 and through the perforations to establish that the formation 11 can be deformed. A pad of the carrier fluid such as lease oil or a fracturing fluid is then injected into the deformed formation at sufficient rate and pressure to displace the formation grains away from the immediate vicinity of the wellbore 10. The viscosity and fluid loss properties of the displacing fluid should be such to prevent excessive penetration or leak-off into the pores of the formation. It is preferred that the fluid be a viscous water-inoil emulsion having a viscosity in excess of about 50 centipoises at formation temperature.

After a sufficient fluid pad has been injected to initiate the deformation forming a cavity adjacent the wellbore, the specially graded pack sand is blended into the stream at concentrations between about I and 4 pounds per gallon. The particle size of the pack sand can be determined by conventional techniques such as those based on the lO-percentile point on the sieve analysis curve of the formation sand. Field experience has indicated that a particle size falling in the 20/60 mesh range will satisfy most well requirements. The slurry is injected into the formation at generally the same rate and pressure causing the cavity to grow generally radially outwardly from the wellbore 10. Experience has shown that at the contemplated pressures and rates the volume of the aggregate injected is almost limitless indicating that the size of the inner and outer filter beds will be a matter of economics rather than physical limitation. Computations based strictly on volumetric and radial displacement are used to determine the volume of the outer filter bed. As the slurry of the carrier fluid and aggregate are injected into the formation II, it is believed that the formation 11 is displaced generally radially outwardly with the carrier fluid filtering into the formation 11 screening out the pack sand on the formation face. Thus at the conclusion of the placement of the pack sand, the sand exclusion zone extends radially outwardly from the wellbore.

After the desired volume of pack sand has been placed in the fluid stream, the gravel is blended in the stream and injection continued. The particle size of the gravel should be such to bridge the pack sand and provide an inner zone permeability at least about 5 times greater than the outer zone permeability. The proper gravel size can be determined by IO-percentile technique described above. However, for most applications an 8/l2 or l/20 mesh sand will provide the desired permeability and effectively bridge the pack sand. The gravel concentration in the slurry can be between 1 and 4 pounds per gallon. Here again the amount of slurry injected is based upon the volumetric displacement of the gravel in forming the inner filter bed. The amounts of pack sand and gravel preferably should provide an outer filter bed having a radial extent of at least 5 wellbore radii and an inner filter bed having a radial extent of at least 2 wellbore radii.

FIG. 3 diagrammatically illustrates the radial extent of the sand exclusion zone 13 and the high permeability zone 14 provided by the inner and outer filter beds, respectively. The wellbore radius is indicated by the arrow 16 and the radial extent of the inner and outer zones 14 and 13 are represented by the arrows l7 and 18, respectively. As evident from the geometries of the zones 13 and I4, fluid flowing from the formation 11 must pass successively through the outer zone 13 and inner zone 14 before reaching the wellbore 10. It should be noted that the configuration of the zones 13 and 14 will vary considerably depending upon several factors including the direction and density of the casing perforations, confining formation stresses, and injection materials and conditions. The important feature of this completion however is the presence of the two zones 13 and 14 separating the formation 11 from the wellbore 12.

The technique for controlling the properties of the carrier fluid and for injecting the slurry can be according to those described in the U.S. Pat. No. 3,378,074, issued to O. M. Kiel and dated Apr. 16, 1968.

With the graded aggregate placed in zones 13 and 14, means must be provided for retaining the aggregate in place during production of the well. This can be accomplished in several ways such as inserting a mechanical device e.g. screen or liner 19, (FIG. 1), or by con solidating a portion of the aggregate in the immediate vicinity of the wellbore (FIG. 2).

Considering first use of the mechanical device (FIG. 1), the slurry containing the gravel is displaced from the tubing or casing by the completion fluid at such a rate and pressure to leave excess gravel in the casing 12. If necessary additional gravel is spotted to provide a packed interval which completely traverses the perforated interval of the casing 12. The perforated liner [9 suspended on the tubing is then washed in place by conventional techniques. Optionally, the liner 19 can be provided with a packer 20 to seal the upper end of the liner-casing annulus. With the liner 19 located, the well is gradually brought in by swabbing or other techniques whereupon formation fluids flush the carrier fluid from the packed zones 13 and 14. The formation fluids pass first through the sand exclusion zone l3 and then the high permeability zone 14. The majority of formation sand entrained in the fluids are filtered out in the sand exclusion zone 14 by screening or bridging.

Formation fines which escape the sand exclusion zone 13 upon entering the highly permeable zone 14 will be carried to the wellbore l0 owing to the inability of the coarse gravel to establish a sand bridge. Since the highly permeable zone 14 embraces the critical flow area, it is important that the flow passages therein remain open. By locating the sand exclusion zone 13 at a position radially spaced from the wellbore l0, and not in the immediate vicinity thereof, the relatively large interstitial flow passages through zone 14 should remain open.

Alternatively, the support can be provided by plastic consolidation techniques. Thermosetting plastics commercially available for sand consolidation treatment include phenol-formaldehyde, furan, and epoxy resins. These plastics can be applied to consolidate the inner zone gravel by a precoated treatment or by injecting the plastic after the inner zone gravel has been placed. In the precoated treatment, the carrier fluid for the gravel includes the consolidating agents. Preferably the plastic treatment should be designed to consolidate at least all of the gravel in the inner zone 14.

Plastic consolidation inherently involves some red uction in permeability because of the plastic filling the gravel pores. However, the permeability of the coarse gravel in zone 15 which generally will be in the order of 500 darcies, can readily accommodate permeability reductions with little effect on well productivity.

Several wells completed in the Miocene formation were gravel packed by the method according to the present invention. All of the wells, treated were problem wells which were incapable of producing for long intervals without standing up.

The procedure for treating the wells was as follows:

1. The well was cleaned out to total depth.

2. The formation was then broken down by pumping water into the formation.

3. A carrier fluid was injected into the formation at such a pressure and rate to deform the formation. The carrier fluid used was an oil-water emulsion frac fluid having a viscosity of about 50,000 centipoises at atmospheric temperature. From 2,000 to 3,000 gallons of the carrier fluid were injected.

. The pack sand (20/30, 20/40 and/or 40/60) U.S. Standard Mesh sand was blended into the carrier fluid at a concentration of from one to four pounds per gallon. The total amount of pack sand injected ranged from l4,000 to 32,000 pounds, depending on the interval being packed. About 3,000 pounds of sand were injected for each foot of perforated interval.

5. The gravel (8/l2 U.S. standard mesh sand) was blended in a resin-diesel oil dispersion at a concentration of one to four pounds per gallon. The

amount of gravel injected ranged from l,000 to 4,000 pounds. About 300 pounds of gravel were injected for each foot of perforated interval. The

resin used in some wells was a furan resin provided by Halliburton Company and marketed under the trade name of Sanfix. Other wells were treated with an epoxy resin provided by Dow Chemical Company. The injection rates and/or concentration were controlled to effect sandout leaving excess gravel in the casing.

The well was cleaned out to total depth.

. A resin catalyst compatible with the type of thermoset used in step was then injected into the formation.

8. The well was closed in for a sufficient period of time to permit the resin to harden.

9. The well was placed on production.

The following table presents treatment and production data for wells gravel packed according to the present invention. All of the wells were perforated casing completions in the Miocene formation.

100X in Now, assume that the completion includes an inner zone packed with 8/12 mesh sand as contemplated by the present invention. The pressure drop is then:

so 100x10 in Treatment Inner sons Outer lone Gale. Cale. Prod.

Mesh Band rsdlus Mesh Band radius rats slss (pound) (lest) sise (pound) (lest) (bbL/day) 8/12 4, M0 1. 8 Will 28, CID 3. 4 [(8 8/12 s, 100 1. 4 /80 23. 600 8. 8 168 8/12 I, am 2. s sum M, M B. 8 M 8/12 2, mo 1. 5 20/0 84, 400 5. 4 92 8/12 8,800 1. 2 sum 82, an 8. B 114 8/12 3, M I. 0 20/00 25, M 4. 8 874 2, 1. s sn so ss,o0o 5.4

of producing for long periods of time because of sand entry into the wellbore. However, following the gravel pack completions, the wells sustained relatively long periods of production and, in some cases, at a stimulated rate. The rates indicated above were stable production rates after treatment.

The following equations illustrate the stimulation effect of the high permeability zone in the wells packed with the 8/12 mesh sand.

Assuming that the well was packed only with the pack sand, the permeability of the 20/30 mesh sand is in the order of I00 darcies. However, as formation fines clog the interstices in the critical zone, the

permeability in the immediate vicinity of the wellbore approaches that of the formation or about l,000 millidarcies. The pressure drop through the packed interval extending three feet radially from the wellbore can be calculated by the Darcy radial flow equation:

0 in 108T];

As previously mentioned, these wells were incapable. well productivity but imparts forces on the consolidated zone which tend to destroy the bonding Will More important than the stimulation effect afforded by the proposed completion is the fact that the large interstitial flow passages through the 8/l2 sand permit the passage of formation fines.

By way of summary then, the present invention offers several advantages over presently-known sand exclusion methods. Specifically, it prevents plugging in the critical flow area of the formation; it provides a sand exclusion zone and a highly permeable zone which should result in well stimulation; and it enables a more efficient application of sand consolidation without harmful reductions in permeability in the consolidated area.

in ame. w

l. A method for gravel packing a well having a casing penetrating a poorly consolidated formation which comprises:

injecting a fluid into said formation at a pressure sufficient to form a cavity around said casing, said cavity extending radially outwardly from said casmg;

packing said cavity with a fine aggregate material to form a first filter bed surrounding said casing, said fine aggregate material being sized to control the migration of formation particles; and

thereafter, packing said cavity with a coarse aggregate material is form a second filter bed surrounding said casing and separating said first filter bed and said casing, said coarse aggregate material and mesh on the U. 8. Standard Sieve Series.

4. The method as defined in claim 1 wherein said coarse aggregate material has a particle size between about 8 and 20 mesh on the U. S. Standard Sieve Senes.

5. The method as defined in claim 1 and further comprising the step of injecting a liquid resin into the second filter bed for consolidating a portion of the coarse aggregate material surrounding said casing. 

1. A method for gravel packing a well having a casing penetrating a poorly consolidated formation which comprises: injecting a fluid into said formation at a pressure sufficient to form a cavity around said casing, said cavity extending radially outwardly from said casing; packing said cavity with a fine aggregate material to form a first filter bed surrounding said casing, said fine aggregate material being sized to control the migration of formation particles; and thereafter, packing said cavity with a coarse aggregate material is form a second filter bed surrounding said casing and separating said first filter bed and said casing, said coarse aggregate material being coarser than said fine aggregate material and being sized to control the migration of said fine aggregate material in said first filter bed.
 2. The method as defined in claim 1 wherein said coarse aggregate material provides a permeability in said second filter bed which is at least five times greater than the permeability provided by the fine aggregate material in said first filter bed.
 3. The method as defined in claim 1 wherein said fine aggregate material has a particle size between about 20 and 60 mesh on the U. S. Standard Sieve Series.
 4. The method as defined in claim 1 wherein said coarse aggregate material has a particle size between about 8 and 20 mesh on the U. S. Standard Sieve Series.
 5. The method as defined in claim 1 and further comprising the step of injecting a liquid resin into the second filter bed for consolidating a portion of the coarse aggregate material surrounding said casing. 